Boat Vibration Problem

Hello,

I have a 1988 20' Four Winns with an OMC sterndrive. Last year the boat was running great until I hit a rock and now have a vibration that exists while in gear or in neutral. When I hit the rock I was at no wake speed and it stalled the engine and did enough damage that I had to replace the prop (aluminum.) As I said the vibration occurs regardless if I am in gear so I know it is not the prop or prop shaft. Yesterday I pulled the lower unit (came out easily) and couldn't see any damage to the shaft or U-joints. (Although I did find a pine needle in between the coupler and gimbal bearing.) I spun the gimbal with my hand and that didn't appear loose and spun freely. I also noticed this still had plenty of grease on it. (I replaced it 2 years ago and checked the alignment at the time which was fine.) I then started the boat with the lower unit off and the vibrations were still present.

I am planning on checking the engine timing, any other suggestions? How do I tell if the engine mounts are bad? What about the coupler could a bolt have bent or be broken? Why would I have a pine needle in their? Is there some type of gap between the flywheel and the coupler that opened up? How would I check for that?

Thanks for any help.

Re: Boat Vibration Problem

coupler is bolted right to the crankshaft so no gap - but I suppose an old coupler might have been distorted with the impact.

while the drive is off, maybe just run the engine and look through the gimbal with a bright light and watch the coupler to see if it is making any sort of orbital motion....just be aware the exhaust is going to be in your face too

Re: Boat Vibration Problem

Ouch... I wouldn't worry too much about the pine needle...

Those rock impacts are rough.. There is a fella on the merc forum I am pretty sure broke his crank from a WOT impact with a rock..

Anyhow I wanta say coupler too.. Something is outta wack to cause a vibration.. Since you removed the outdrive we know its not that...

Re: Boat Vibration Problem

Thanks for the help. Today I checked the timing and unplugged each spark wire and found the timing is set correctly and is advancing. The RPM's dropped with each plug disconnected so all cylinders are firing.

I tried watching the coupler but it is hard to tell about the orbiting. The last piece in side is a rusty color with a wear spot where the end of the shaft went against it. (I am assuming this is the fly wheel) The wear spot is not a perfect circle so when it goes around it does show an orbital pattern but I think this is just how I am viewing it. Should I try and insert the alignment tool next? (I need to borrow this.) If the couple is bad would I be able to see this with the tool? Most of what I have read about couplers going indicate a power problem, burning smell and high pitched whine. I don't have any of those. I also checked the front mounting bolts for the engine and their is no play so I am assuming those are good. It is difficult to get to the back ones but I will check later. Any other suggestions of things to try?

Thanks again.

Re: Boat Vibration Problem

Should I try and insert the alignment tool next?

Click to expand...

Actually, Mercury and others suggests pulling the drive and checking the alignment yearly.

Vibration is VERY hard to diagnose. Check the propshaft for runout. You should be able to see it if it's out very much.

You would know if your coupler had failed. The boat wouldn't go.

Make sure your engine mounts, transom plate, and other mount-points are tight. Look for "slop" in the steering. See if the vibration changes when you trim up or down a little or turn from left to right......

Re: Boat Vibration Problem

Check your alignment. Also, if you have a transom that is getting soft from rot, the impact may have moved your transom assembly slightly.